Ultra short light pulse generation



Nov. 3, 1959 H. FISCHER 2,911,567

ULTRA SHORT LIGHT PULSE GENERATION Filed March 28, 1958 4 Sheets-Sheet 1"III! IIIIIII-HIVIIL INVENTOR. HEM Z F/J' (W51? H. FISCHER 2,911,567

4 Sheets-Sheet 2 V/v/ z ULTRA SHORT LIGHT PULSE GENERATION Nov. 3, 1959Filed March 28, 1958 INVENTOR.

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ULTRA SHORT LIGHT PULSE GENERATION Filed March 28, 1958 4 Sheets-Sheet sT/IIE-F-F. I I I I I I I l l I T/PKGSISWEE mzzr INVENTOR. f/Z/AIZ F/JZAER Nov. 3, 1959 H FISCHER ULTRA SHORT LIGHT PULSE GENERATION 4Sheets-Sheet 4 Filed March 28, 1958 w w 4 H United States Patent2,911,567 7 ULTRA SHORT LIGHT PULSE GENERATION Heinz Fischer, Belmont,Mass. Application March 28, 19 5s, Serial No. 724,776 2 Claims. oi.315-61) (Granted under Title 35, US. Code (1952), sec. 266) Theinvention described herein may be manufactured and used by or for theUnited States Government for governmental purposes Without-payment to meof any royalty thereon.

This invention relates to lightgeneration, and particularly to thegeneration of light pulses of extraordinary brilliance and having aduration'limited to a small fraction of a microsecond.

In my Patent No. 2,728,877, granted December 27, 1955, there isdescribed a method and apparatus for generating heat pulses of highintensity, suitable for many purposes including, as examples, infra-redsignalling, nuclear reaction triggering, material melting, and metalvaporization. As summarized in the specification of the patent, themethod comprises building up a direct current charge on a capacitorassembly ofvtoroidal contour, and discharging the capacitor across apressurized gas gap at the axis of: the toroid, the gap being in thecenter of a chamber whose walls are opaque except fora small insert oflithium fluoride or some equivalently good conductor of opticalradiation. The capacitor and associated circuitry are such as tominimize inductance and resistance effects and thus reduce thedischarging time by eliminating practically all current flow delayingfactors.

While the objective discussed at greatest length in the aforesaid patentis the rapid production of an extremely high temperature, the fact isthat the production of intensely brilliant light pulses of ultra-shortduration is also inherent in the operation, and references to this factare set out in the patent specification. The present invention involvesthe provision of three additional methods of producing ultra-short lightpulses of extreme brilliance, which methods may be distinguished, eachfrom the others by these designations, namely:

(l) The open air gap method;

(2) The low-pressure method; and

(3) The high-pressure method.

The present invention further involves the provision of apparatuscapable of putting into practice the concepts of light-pulse generationfalling into the three categories indicated.

Other objects and characteristics of the invention will become apparentupon reference to the following description of the invention and theaccompanying drawings wherein:

Figs. 1 to 5, and Fig. 13, are views, partly sectional and partlyschematic, illustrating structures for putting into practice the openair gap concept above referred to;

Figs. 1 6, 7, and 8 are views illustrating structures for Figs. 10, 11,and 12 are pulse time diagrams hereinafter explained.

By way of introduction to the following description it may be noted thatcommercial flash lamps utilizing capacitor circuitry obtain light pulsesof approximately 1 microsecond or longer. Their peak brightness inabsolute values of brightness, i.e., candles per cm. are normally notreported, but have been measured by applicant recently and found to beinthe order of approximately 5 million candles/cm? Experimental studieshave proved that it is the length of the electrical pulse of such flashlamp which in the first approximation determines the length of the lightpulse, and it was observed by applicant'that the brightness maximumcorrelates closely in time with the current maximum even in case ofcapacitor discharges as short as 5-10 seconds. It also was found thatthe amplitude of the light pulse (brightness) is determined to a largeextent by the discharge current.

In case of a capacitor discharge in which the resistance in the sparkgap is not kept up by a squeeze of the spark channel, the resistance ofthe gap during the time of the discharge becomes extremely small, whichresults in an oscillating discharge. Here the maximum current in firstapproximation is expressed by the following equation:

1) i U c/L) 1/2 [amps] the pulse length on the other hand is So, it isobvious from (1) and (2) that at a given breakdown voltage U the ratioC/L must be made as large as possible in order to get a current I' aslarge as possible (i.e., maximum. brightness) at a minimum pulse lengthr.

The ideal case, in respect to current pulse shape and energy transferfrom the capacitor into the gap, is met when the inductance L can bedecreased to a value (3 I L=4R2C because here the discharge iscritically damped and the energy transfer into the channel beingmaximum, which is /2 of the total capacitor energy.

(4) K=CU /2 [joules] So far, the critically damped case could not bematerialized ina high current capacitor discharge without squeezing thechannel through a tube or aperture. Actually, the spark resistance R(decreasing with the current) may become of the order of less than 10-ohms representing only a' small fraction of the total resistance R. Inthis case, most of the energy is used up in the outer circuit.Consequently, reduction of the resistance R of the outer circuitrto aminimum is of importance. 3

The oscillating spark current leads to humps and a tail in the decayingpart of the light pulse and in most T2- (LC) [sec] [henries]applications it is required that this tail is kept to a minimum,respectively have been cut off; this latter is accomplished to someextent by an additional quenching gap.

The spark resistance can be kept up to some extent by forcing thedischarge through a narrow capillary or other aperture, which stops thechannels expansion. The problem is to squeeze the channel just right sothat the circuit is critically damped, which is when The proper diameterto enforce this condition depends. upon discharge parameters such asgas-pressure,

TABLE 1 Brightness B and pulse length At [Fig. 1 type of lamp] Cap,Hole, U, 0, L, 1, tum, B, 10 A t A t,, R, K, N0. cm. cm. Kv 1. H. #580.C/L K. amp. candJ sec. sec. ohms joules cmfl- X10- .07 0.1 3.17 0.10.004 0.127 15. 7 20. 2 36 0. 5 08 0. 1 3. 33 0. 1 0. 004 0. 127 25 18.6 18. 7 0. 2 0.37 43 0. 55 08 3. 5 0. 1 0. 004 0. 125 25 17. 5 23. 2 0.25 0.6 0. 55 08 0. 1 3. 33 0. O5 0. 004 0. 091 12. 5 11. 8 11. 9 -0. 220. 54 0. 275 07 0. 1 3. 17 0. 025 0. 004 0. 063 6. 25 7. 9 5. 5 -0. 01O. 38 50 0. 124 .07 3.1 2.8 0.035 -1. 95 80 i 27.8 42 13.4

current, etc., and is a delicate problem to determine. Theexpansionforces, on the other hand, especially at large gas pressure may becomeso large that a practical solution of the squeeze problem becomesdifl'icult.

Fig. 1 represents the diagram of a simple air lamp. It consists of acoaxial capacitor 1 which surrounds the discharge 2, as indicated. Thus,the capacitor, the air gap and the electric connections 3 to the gapform together a symmetrical coaxialline reducing the inductance L of thecomplete circuit to a minimum. The discharge takes place between thepointed electrode 4 and the center hole 5 in the top plate, producing abright spark channel between the electrodes which'may extend into aradiating bubble as indicated in Fig. 1. The size and brightness of thisbubble depends largely upon parameters such as of the nature of gas,pressure, capacity C, breakdown voltage U, hole diameter d, as well asthickness of the top plate. The bubble extends out of the hole as muchas /2 cm. and more. Change in polarity of the electrodes 4, 5 does notseem to greatly influence the visual appearance of the bubble, however,the amplitude (brightness) of individual shots was found to be morereproducible in case of a negative top plate (cathode). The insideconnection to the pointed electrode provides several exhaust holes 6, asindicated.

A triggered coaxial air gap is shown in Fig. 2. It applies the samegeneral geometry, however, provides a trigger 7. The pointed electrodein Fig. l is now replaced in Fig. 2 by an electrode having a hole 4through which the discharge between electrodes 4 and 5 may be triggered.This hole in electrode 4 is important since it permits short waveradiation from the trigger discharge to enter into the prospective pathof the main discharge in chamber 2 which is to be triggered. Thisradiation from the trigger discharge has been found to cut downformative time lags (jitter) in the firing of the main discharge. Thetrigger pulse at electrode 7 may be positive or negative.

The end-on geometry, as applied in Figs. 1 and 2, provides basically apoint light source, the radiation of which originates from the bubbleformed in the hole of the top electrode. Added to this radiation is theradiation from the spark channel which is seen end-on through the centerhole in case of a transparent bubble. Results obtained with the open airgap method, as recorded below in Table 1, indicate that the bubbleapparently contributes largely to the radiation, The ra;

Inductances.-The toroidal capacitors, used in the tested lamps were ofthe extended foil type indicated in Fig. 1. These values of L, however,do not represent optimum values for this type of capacitor. In case oftest specimens Nos. 4, 5 in Table 1, the capacitors wereoverdimensioned; actual L values can be made smaller, at least by afactor 2 to 4, withoutany change in material or deslgn.

Substantial reduction of L under that of the extended foil-typecan beachieved by substituting a long ribbon type coaxial capacitor. In suchan arrangement the capacitor foils need not be cross-conected at top andbottom; one end may be left open and the other end fed directly into acoaxial line, which is connected to the sparkgap in the manner indicatedin Figs. 1 and 2. Inductances of L l( henries may be safely expectedfrom this type of capacitor.

Brigl1tness.-.-The B values already obtained with the coaxial open airgap are considerably larger than those of the customary commercial flashlamps as demon strated in Table 1. With the tested lamps, the brightnessgoes roughly with the current i as was discussed in the introduction,providing thatL and C/L factor determine B to a large extent. Thesignificance of extremely good contact is demonstrated in the data oflamp No. 2 in Table 1, where B is reduced due to increased resistance]&. in the outer circuit, in spite of C and L being equal to No. 1; thepulse width on the other hand is reduced substantially by the increaseddamping.

On the basis of these results, approximate B values for the inventedlight source may be predicted from known electric data, U, C, L, whichmay be calculated. The calculation of L, however, becomes difficult whenconsidering extreme small values L 10" henrics and consequently shortelectric pulses. Here I13 spark gap itself may represent a considerablepart of the total inductance.

With increasing spark current, the brightness approaches a limitingvalue which in the open air gap was found to be between 40-50 millioncandles/cm. (see also lamp 6 in Table 1). When this value is reached,the radiation pulse (brightness as a function of time) be comes wider.This widening shows up much more pronounced in the total flux (candles)time function. It may even lead to a considerable increase of the fiuxamplitude after the current has reached its maximum.

Pulse length -Applying geometry of Fig. 1, the experilava, etc.,

*3 mental length of the radiation pulse, according to Table 1, isroughly At1-27 for the halt width and approximately At -6r for the basewidth including the tail which results from the current oscillations-Atisdifiicult to analyze since the bubble and spark channel may havedififerent decay times. I In case of a transparent bubble, the tail ofthe radiation time function as emitted through the hole may even be cutoff to a large extent by its aperture, as is discussed in a laterparagraph. Since the opacity of the bubble is expected to decrease withthe spark current, this maymean that pulse widths At 6T may be expectedin case of extremely small 0.1 microsecond because C/L (seeintroduction) and consequently i will have to be sacrificed in order toobtain extremely small values of I.

Predicted values-Based on the discussion of the preceding paragraph, thefollowing values of B and pulse length At are considered possible:

Open cage-The coaxial pulse light source discussed so far is notrestricted to end-on observation (see Figs. 1 and 2) The open air gapmay be extended out of the coaxial capacitor by means of an open cageallowing completeaxial side view of the channel, as illustrated in Fig.5. Such a lateral display of the lift pulses may be advantageous forcertain purposes.

Sealed coaxial low-pressure lamp.

(1) The geometry which was discussed in the precedg paragraphs for theopen air gap may be applied for the sealed low-pressure lamp. Fig. 6shows the design of such a lamp as built and tested successfully to amoderate extent. This lamp does not suggest maximum pulse power becauseof the danger of window blackening. Definitive advantage however lies atsmall pressure in the lower breakdown voltages, longer spark channel,easier squeeze and higher repetition rates. The lamp may apply any inertgases;

TABLE 2 Predicted values of B and At K, tu Rap, B, 10* A A t CapacitorU, Kv. O, F. joules L, pH. C/L 1', see amp. candl es/ uses. see.

4 0. 2 1. 6 004 50. 0 178 28. 2 283 -40 356 1. 068 Extended toil 4 0. 4002 25. 0 063 20. 0 40 12 376 4 0. 02 16 001 10. 0 028 12. 6 634 -18 056168 4 0. 2 1. 6 001 200. 0 089 56. 5 141 -40-50 18 534 Long ribbon 4 054 0005 100. 0 031 40 20 -40 06 186 4 02 16 0005 40. 0 020 25. 4 316 04120 These values of Table 2 are to be considered rough estimations. Ofspecial interest appears the application of the long ribbon typecapacitor, since it possibly allows a critical damped discharge (see lowvalues of R in Table 2).

The amplitude (brightness) of individual shots as observed from end-on(see Figs. 1 and 2) is connected with statistic changes largely due tothe fact that the discharge does not fire each time from the same spotof the pointed electrode into the same spot of the hole. Improvement inthe reproducibility is obtained by radioactive material used'inelectrode tips 4, 7, and walls of the hole electrode.

Most successful in accomplishing reproducible amplitudes of single shotshas proven an arrangement as sketched in Fig. 3, however, leads to aslightly wider light pulse (see Table 1). Here the discharge fires fromelectrode 4 to another tip which is extended into the gap from the topplate, as indicated in Fig. 3. The hole is set off center. Seen throughthe hole, the channel now lies across. The discharge does not fire anymore axially into the hole, eliminating the bubble to a large extent.vThismeans the channel is seen perpendicularly across which accounts forthe longer radiation pulse, as illustrated in the graph, Fig. 10; L and'r are practically unchanged due to the unchanged overall geometry. Fig.3a shows a variation of the same concept.

The trace depicted in Fig. 10 consists of 10 shots, demonstrating thatthe amplitudes are constant within approximately :L5%. The Fig. 10 timebase is 0.2 psec/div.

Squeeze of the channel by means of an inserted capillary 10a made ofinsulating material such as quartz, fired is shown in Fig. 4. Fig. l3shows another form of squeezed channel.

This arrangement not only allows increase in the spark resistance, Rpreferably to the critical value, see

Formula 3a, but also keeps the channel lined up in the axis of thecenter hole leading to a constant amplitude of individual shots.

, visual brightness B increases Thecontainer 9 in Fig. 6 consists ofglass, quartz glass or other insulating material. The electrodes 2 aremade of cover with inserted tungsten pieces 3 for the electrodes. Thebubble Window 8 due to its extremely small thickness had adequate U.V.andinfrared transmission, allowing the lamp to be used for visual aswell as for U.V. and infrared source. Undesired'radiation may befiltered out by spectral filters.

The pressure in the tube can be chosen anywhere around one atmosphere orunder, depending upon the application in this specific case and thenature of gas. Generally, a gas pressure in the range from -400 mm. Hgmay be considered practical.

(2) Simple sealed lamps with a squeezed channel are illustrated in Figs.7 and 8.

Demountable high-pressure lamp.-

(1) Different designs of coaxial high pressure lamps. have been builtand successfully tested. They follow the geometry of the lamps describedabove. They include the end-on type as well as the open cage sideviewtype (see Figs. 1, 2, 6).

(2) Maximum brightness. It has been found by applicant that for a givencapacitor and gap length, the

sure p to an ultimate upper mined mainlyby thena'ture V creased to someextent by improved efliciency factor C/L. Opacity measurements showedthat this ultimate brightness is obtained shortly after the sparkchannel reaches opacity. So far, the largest valuesB g were obtained inhelium, amounting to approximately million candles/cul Apparently thisis due to the fact that helium reaches opacity at considerably largervalues of p and current than other gases. Helium, on the other hand, dueto its larger transparency at lower pulse energy, is by no means themost eificient radiator. Argon, for example, has much largerbrightnessat relatively small energies. vTable 3 gives comparison valuesof brightness for ditferent gases in diiferent type ofapparatus.

limit R which is deterof the gas; B can be in strongly with the gaspres- TABLE 3 Comparison of values of'brightness obtained in difieremapparatus M M di K No. Ga Gas P 'U C, L, 1, ,O/L i illion 1 1- 'crn i"To Kv. 12F. H. sec. K231 Cand ohms oules 0.2 Ar 25 4.5 0. 57 .15 1.953.3 8.8 11.5 12.0 5.8 0.2 Ar 100 8.5 0.57 .15 1. 95 3.8 16.6 23.0 7.020.5 0. 2 He 200 4 75 0.57 15 1. 95 3. 8 9. 3 6. 12. 0 6. 4 0.2 He 1,500--16.0' 0.57 .15 1.95 3.8 31.2 60.0 4.0 73.1 0.2 Ar 20 3.15" 2.75.033 1. 95 82.5 28.6 72.5 2 13.0 0.2 Ar 40 40 2.75 .033 1.95 82.5 36.298 2 20.8 0. 2 He 100 2.66 .2. 75 033 1. 05 82. 5 24. 2 3. 9 2 9. 7 0. 2He 200 4 2. 75 033 1. 05 82. 5 37. 7 8. 1 2 23. 6 0; 2 He 400 6.8 2.75.033 1. 05 82.5 61.8 i 170 2 63.5 0. 2 Ar 50 2; 5 5. 5 016 1. 90 350 46.7 23 17. 2 0. 2 He 100 2. 5 5. 5 .016 1. 90 350 46. 7 45 17. 2

1 10, 11 and on through hole, all others side view. 1 Ultimatebrightness.

If this explanation is true, there is promise that higher ultimatevalues B may be obtaine'd'in hydrogen which is more transparent than"helium.

(3) Medium energy. A compact and easily demountable test lamp forpressures between approximatelyl-Zt) atmospheres and breakdown voltagesunder approximately l0 kv. is shown in Fig. 9. This lamp, which has beenextensively and successfully testedin the gases- HL, He and argon, ismounted in a container which encloses both the capacitor and the gap;however, only the spark chamber is pressurized.

The spark chamber 51 is bounded peripherally by a sealing gland 52 of"Ieflon or the like and is bounded laterally by electrodes 53 and 54,the former being the centrally perforated portion of capacitor terminalplate 55, and the latter being the grid-like central section of theother capacitor terminal plate 56, which plates 55, 56 enclose andcomplete the capacitor assembly 57. The central section 54 of plate 56connects with'the base portion by way of cylindrical section 58 fittingaround the supporting spool 59 of insulating material. -In fact, parts56 and'58 may bemetal-plated on-spool-59 and its flange-60, withtheperforated central "grid 54 bonded thereto. Spool 59 is centrallybored toform a passage 61 for admission of gas under pressure tO-chamber51 by way of the perforations in grid 54. A quartz window 62 is insertedin a gland element 63 threadedly engaging pressure plate 64, which inturn is adjustably mounted on flange 65 of housing 66 by means of screws67. Teflon seals 68 and 69 are compressed by the pressure, appliedthrough adjusting elements 63 and 64, respectively, to elfectively sealthe chamber 51. Current is supplied to capacitor terminal plate 56 byleads 71 and 72, the former connecting to a source, not shown. Thecircuit is completed to ground 73 by way'of terminal plate 55 andhousing 66. A second insulator 74 surrounds capacitor 57 and spool 59.

. The Fig. 9 construction can be built in extremely small sizes andfavorable C/L ratio. Brightness values of approximately million candleswere observed inendon position in argon, and relatively easily may bepushed to the maximum value'B ofapp'roximately 40 to 50 millions withincreased p, U and C/Las discussed in the precedingparagraph. So far thedesign of this particular lamp was limited to C2025 at". and C/L' 25,which is comparable to theelectrical data of the open air gap asdescribed in Table 1.

Radiation time functions investigated in-the gases H and heliumrevealedthat the. decay'time of the pulse differs strongly in differentspectral'regions. .Fig. 11 and Fig. 12 show radiationtime functionsinhelium at two different spectral ranges. The electrical data werelengths 4685 A. and 5875 A.

It has been found that the radiation pulse in Fig. 11 is considerablyshorter than the current pulse, the latter being oscillating. Thislatter effect may be explained by the assumption that the spark channelhas a dark core in this particular spectral range. Thus the expansion ofthe channel as a function of time would move the radiating shell out ofthe angle of view as defined by the center hole in the top electrode.This results in this sharp triangular light pulse with cut-off tail. Thetime base in Fig. 11 is 0.5 microsecond per division, which makes thehalfwidth of the radiation approximately 0.25 ms. and the base width 0.5ms. This in relation to the pulse length '1' is much shorter than thatof the open airgap in Table l.

The rather ragged and much slower decay in'Fig. '12 indicates thatradiation of thisparticular spectral range (5875 A.) must be filling upthe otherwise dark core of the channel. This radiation, of course, couldalso come from the bubble. A very high modulation frequency of thisradiation, amounting to roughly 10 megacycles, is indicated. Thisphenomenon may be related to a magnetic hydrodynamic oscillation withineither the interior of the channel or the bubble in the center hole.

What is claimed is:

1. Apparatus for generating lightpulses comprising a capacitor assemblyin the form of a torus, to provide a central space of cylindricalcontour, said capacitor assembly including terminal plates extendingacross said central space, said plates having aligned apertures alongthe axis of said assembly, means for storing current in said capacitorassembly, and means positioned on said axis for triggering the dischargeof current stored in said capacitor assembly to provide short waveradiant energy serving to reduce the jitter factor in said currentdischarge.

2. Apparatus for generating light pulses comprising a capacitor assemblyin the form of a torus, to provide a central space of cylindricalcontour, said capacitor assembly including terminal plates extendingacross said central space, said plates having aligned apertures alongthe axis of said assembly, means for storing current in said capacitorassembly, and means positionedbeyond the area bounded by said terminalplates, for triggering the discharge of current stored in said capacitorassembly to provide short wave radiant energy serving to reduce thejitter factor in said current discharge.

References Cited in the file of this patent UNITED STATES PATENTS2,728,877 Fischer Dec. 27, 1955

